With its low cost and wide availability, lignocellulosic biomass has been under worldwide-study as a feedstock for renewable liquid biofuels. One impetus, in particular, is that biomass derived fuels have zero net CO2 emissions if produced without the use of fossil fuels. However, lignocellulosic biomass is not currently commonly used as a liquid fuel source because typical current conversion processes are not considered to be economically feasible. Several routes are being examined to convert solid biomass to a liquid fuel. At low temperatures (e.g., 200-260° C.) diesel range alkanes can be produced by a multi-step aqueous-phase processing (APP) of aqueous carbohydrate solutions involving dehydration, aldol-condensation and dehydration/hydrogenation (G. W. Huber, J. A. Dumesic, Catalysis Today 2006, 111, 119-132). However, APP requires that solid lignocellulosic biomass first be converted into aqueous carbohydrates. At higher temperatures (˜800° C.) solid biomass can be reformed to produce synthesis gas through partial oxidation over catalysts in an auto thermal packed bed reactor. (P. J. Dauenhauer, J. D. Dreyer, N. J. Degenstein, L. D. Schnudt, Angew. Chem. Int. Ed. 2007, 46, 5864-5867). The synthesis gas produced from the reaction can be fed to a secondary process to make fuels and chemicals. For certain applications, an ideal process for solid biomass conversion may involve the production of a liquid fuel that fits into existing infrastructure from solid biomass in a single step, at short residence times. Unfortunately, neither the APP nor syngas process meets such criteria.
Another approach for biofuel production is fast pyrolysis, which can involve, for example, rapidly heating biomass (e.g., ˜500° C./sec) to intermediate temperatures (e.g., ˜400-500° C.) followed by rapid cooling (e.g., residence times 1-2 s). (See, A. V. Bridgwater, Fast Pyrolysis of Biomass: A Handbook Volume 2, CPL Press, Newbury, UK, 2002.) Conventional fast pyrolysis often produces a thermally unstable liquid product mixture called bio-oils, an acidic combustible liquid mixture of more than 300 compounds that degrades with time. However, bio-oils are not compatible with existing liquid transportation fuels, such as gasoline and diesel, and yields are low.
Accordingly, there remains an on-going search in the art for an economical, efficient route for the production of useful biofuels and related compounds from solid biomass.